1. Technical Field
The present invention relates generally to thermal imaging systems, and in particular to a system for electronically compensating for changes in system imager lens focal length.
2. Discussion
Scanning thermal imaging systems are used in a variety of applications, including surveillance systems and target detection/recognition systems. Such systems typically incorporate a telescopic lens assembly coupled to a scanner. The scanner scans energy from a scene through an imager lens assembly onto a detector array having a plurality of photoelectrically responsive detector elements perpendicular to the scan direction. Each of these detector elements provides an electric signal proportional to the flux of infrared light on the particular detector element. Electric signals generated from the detector elements are subsequently processed by system sensor electronics to create an image that is displayed on a system output device. To improve sensitivity, some of these systems incorporate detectors parallel to the scan direction. The output of these detectors are delayed in time from each other such that, ideally, the scanned image is output simultaneously on all of the parallel detectors. The delayed outputs are then summed (integrated). This process is referred to as time delay and integrate (TDI).
In the above-mentioned thermal imaging systems with TDI, the correct amount of time delay is crucial to a high resolution video signal output. If the delay is not correct, the instantaneous summed output will consist of information from different parts of the scene thereby blurring the information. Moreover, many of the detectors with TDI incorporate multiple subarrays of detector elements. The signal processing electronics must remove the time separation of the images caused by the spatial separation of the subarrays by delaying the output of one or more of the subarrays. The proper delay is related to the spacing between the parallel detectors and subarrays, the scan velocity, and the imager effective focal length. This relationship must be held constant for optimum performance. Unfortunately, there can be variations in both the imager effective focal length and scan velocity due to manufacturing tolerances and/or environmental effects (e.g. temperature) that, unless compensated for, will degrade performance.
In traditional thermal imaging systems with TDI, various methods have been used to ensure the correct amount of time delay. To ensure the correct imager effective focal length, mechanical devices have been implemented in the system to move imager components to compensate for imager lens property fluctuations with temperature. These devices can be either active, such as a motor, or passive, such as a material that has specific thermal expansion properties that compensate for the thermal properties of the imager lenses. However, such traditional thermal imager lens compensating devices tend to increase system complexity, and thus cost, due to the additional associated components and engineering design involved in implementing these devices. Reflective optical elements also have been used to prevent changes in the imager effective focal length over temperature since this type of optics has little temperature sensitivity; however, reflective optics are often difficult to package in the tight volume available for many of these thermal imaging systems.
What is needed then is an electronically implemented focal length compensator that compensates for both changes in temperature and manufacturing tolerances, thereby eliminating the need for the aforementioned conventional mechanically-implemented focal length compensators or reflective optics and thereby minimizing system cost, complexity and system volume requirements.